U.S. patent number 9,654,187 [Application Number 14/296,273] was granted by the patent office on 2017-05-16 for efficient uplink transmission of channel state information.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson (publ). The grantee listed for this patent is TELEFONAKTIEBOLAGET L M ERICSSON (PUBL). Invention is credited to Jung-Fu Cheng, Dennis Hui, Leonid Krasny, Kambiz Zangi.
United States Patent |
9,654,187 |
Hui , et al. |
May 16, 2017 |
Efficient uplink transmission of channel state information
Abstract
A User Equipment in a wireless communication network includes a
multiplicity of antennas, from which a subset of antennas is
selected using a selection scheme synchronized to the network. A
set of sub-carriers is selected from a plurality of sub-carriers
using a selection scheme synchronized to the network. The UE
receives a plurality of known reference symbols over the selected
set of sub-carriers and through the selected subset of antennas. A
frequency response for each selected sub-carrier is estimated over
only the selected subset of antennas. The results are quantized and
transmitted to the network on an uplink control channel.
Inventors: |
Hui; Dennis (Sunnyvale, CA),
Cheng; Jung-Fu (Fremont, CA), Zangi; Kambiz (Chapel
Hill, NC), Krasny; Leonid (Cary, NC) |
Applicant: |
Name |
City |
State |
Country |
Type |
TELEFONAKTIEBOLAGET L M ERICSSON (PUBL) |
Stockholm |
N/A |
SE |
|
|
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ) (Stockholm, SE)
|
Family
ID: |
51569100 |
Appl.
No.: |
14/296,273 |
Filed: |
June 4, 2014 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140286267 A1 |
Sep 25, 2014 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13439759 |
Apr 4, 2012 |
8923148 |
|
|
|
12555966 |
Jun 26, 2012 |
8208397 |
|
|
|
61172484 |
Apr 24, 2009 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
1/0019 (20130101); H04B 15/00 (20130101); H04B
17/364 (20150115); H04B 7/0874 (20130101); H04L
1/0026 (20130101); H04B 7/0691 (20130101); H04B
17/336 (20150115); H04B 7/0626 (20130101); H04L
1/20 (20130101); H04B 7/024 (20130101); H04L
1/0003 (20130101); H04L 25/0224 (20130101); H04L
25/0212 (20130101); H04L 25/022 (20130101); H04B
7/0606 (20130101); H04L 1/0009 (20130101); H04B
7/0639 (20130101) |
Current International
Class: |
H04B
7/02 (20060101); H04B 15/00 (20060101); H04B
17/364 (20150101); H04B 17/336 (20150101); H04B
7/06 (20060101); H04L 1/20 (20060101); H04B
7/08 (20060101); H04B 7/024 (20170101); H04L
1/00 (20060101); H04L 25/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1 786 133 |
|
May 2007 |
|
EP |
|
2004-266814 |
|
Sep 2004 |
|
JP |
|
2007-143143 |
|
Jun 2007 |
|
JP |
|
WO 2008/052479 |
|
May 2008 |
|
WO |
|
Other References
Abe K. et al., "A study on Reduction of the Amount of CSI Feedback
in MIMO-OFDM Eigenbeam-Space Division Multiplexing Systems", The
Institute of Electronics, Information and Communication Engineers
Technical Report, Feb. 27, 2008, vol. 107, No. 518, pp. 293-298.
cited by applicant .
Motoyoshi K. et al., "A study on CSI Feedback Bandwidth Reduction
Method for FDD-based OFDM Systems", The Institute of Electronics,
Information and Communication Engineers, communication society
meeting papers 1, Sep. 7, 2005, p. 428, B-5-28. cited by applicant
.
Duel-Hallen A. et al., "Long Range Prediction and Reduced Feedback
for Mobile Radio Adaptive OFDM Systems", Wireless Communications,
IEEE Transactions, Oct. 2006, pp. 2723-2733. cited by applicant
.
Wong I. et al., "Joint Channel Estimation and Prediction for OFDM
Systems", Global Telecommunications Conference, GLOBECOM '05, IEEE,
Dec. 2, 2005, vol. 4, pp. 2255-2259. cited by applicant .
Kayama H. et al., "A Study on the Accurate Channel Estimation
Method Applying Sinc Function Based Channel Replica for a Broadband
OFDM Wireless Communication System", The Institute of Electronics,
Information and Communication Engineers Technical Report, Aug. 16,
2007, vol. 1 07, No. 192, pp. 99-104, RCS2007-70. cited by
applicant .
Li Y. et al., "A Novel Scheme for Channel State Information
Feedback in MIMO-OFDMA System", Circuits and Systems for
Communications, 2008 ICCSC 2008. 4.sup.th IEEE International
Conference, May 28, 2008, pp. 205-209. cited by applicant .
3GPP TS 36.213, v8.5.0 (Dec. 2008), 3rd Generation Partnership
Project; Technical Specification Group Radio Access Network;
Evolved Universal Terrestrial Radio Access; Physical layer
procedures (Release 8), Section 7.2.1. cited by applicant.
|
Primary Examiner: Pham; Chi H
Assistant Examiner: Huang; Weibin
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-in-Part of U.S. patent
application Ser. No. 13/439,759 filed Apr. 4, 2012, which is a
Continuation of U.S. patent application Ser. No. 12/555,966 filed
Sep. 9, 2009, which claims the benefit of U.S. Provisional Patent
Application No. 61/172,484, filed Apr. 24, 2009, the disclosures of
which are incorporated herein by reference.
Claims
What is claimed is:
1. A method of reporting Channel State Information (CSI) by a User
Equipment (UE) operative in a wireless communication network in
which downlink data is modulated onto a plurality of sub-carriers,
each having a different frequency, wherein the UE includes a
multiplicity of antennas, and the method comprises, at each
iteration: selecting a subset of antennas from the multiplicity of
antennas using a selection scheme synchronized to the network;
selecting a set of non-uniformly spaced sub-carriers from the
plurality of sub-carriers using a selection scheme synchronized to
the network; receiving a plurality of known reference symbols over
the selected set of sub-carriers and through the selected subset of
antennas; estimating a frequency response for each selected
sub-carrier over only the selected subset of antennas; quantizing
the estimated frequency responses; and transmitting the quantized
frequency responses to the network via an uplink control
channel.
2. The method according to claim 1, wherein the multiplicity of
antennas are closely spaced.
3. The method according to claim 1, wherein the step of selecting
the subset of antennas includes selecting a subset of non-uniformly
spaced antennas.
4. The method according to claim 3, wherein the step of selecting
the subset of non-uniformly spaced antennas includes utilizing a
pseudo-random sequence by an antenna selector to select the subset
of non-uniformly spaced antennas.
5. The method according to claim 4, wherein the pseudo-random
sequence is a predefined sequence known to both the UE and the
network.
6. The method according to claim 5, wherein the antenna selector
pseudo-randomly selects a plurality of subsets of antennas, each at
a different time instance.
7. The method according to claim 6, further comprising determining
by the network, which antennas are selected by the UE in each
subset of antennas based on the known pseudo-random sequence and a
sequence seed corresponding to the time instance when each subset
is selected.
8. A method of reporting Channel State Information (CSI) by a User
Equipment (UE) operative in a wireless communication network in
which downlink data is modulated onto a first plurality of
sub-carriers, each having a different frequency, wherein the UE
includes a multiplicity of antennas, and the method comprises, at
each iteration: selecting a subset of antennas from the
multiplicity of antennas using a selection scheme synchronized to
the network; receiving a plurality of known reference symbols over
a subset of the first plurality of sub-carriers and through the
selected subset of antennas; estimating a frequency response for
each of a second plurality of sub-carriers over only the selected
subset of antennas; collecting the estimated frequency responses
into a vector; selecting a group of frequency responses by
multiplying the vector of frequency responses by a mixing matrix
that is synchronized to the network, to yield a vector of selected
frequency responses; quantizing the selected frequency responses;
and transmitting the quantized frequency responses to the network
via an uplink control channel.
9. A method of reporting Channel State Information (CSI) by a User
Equipment (UE) operative in a wireless communication network,
wherein the UE includes a multiplicity of antennas, and the method
comprises, at each iteration: selecting a subset of non-uniformly
spaced antennas from the multiplicity of antennas using a selection
scheme synchronized to the network; receiving through the selected
subset of non-uniformly spaced antennas, a radio signal comprising
a plurality of known reference symbols; estimating a frequency
response for each of the selected subset of non-uniformly spaced
antennas; quantizing the estimated frequency responses; and
transmitting the quantized frequency responses to the network via
an uplink control channel.
10. The method according to claim 9, wherein the multiplicity of
antennas are closely spaced.
11. The method according to claim 9, wherein the step of selecting
the subset of non-uniformly spaced antennas includes utilizing a
pseudo-random sequence by an antenna selector to select the subset
of non-uniformly spaced antennas.
12. The method according to claim 11, wherein the pseudo-random
sequence is a predefined sequence known to both the UE and the
network.
13. The method according to claim 12, wherein the antenna selector
pseudo-randomly selects a plurality of subsets of antennas, each at
a different time instance.
14. The method according to claim 13, further comprising
determining by the network, which antennas are selected by the UE
in each subset of antennas based on the known pseudo-random
sequence and a sequence seed corresponding to the time instance
when each subset is selected.
15. A User Equipment (UE) operative in a wireless communication
network in which downlink data is modulated onto a plurality of
sub-carriers, each having a different frequency, the UE comprising:
a multiplicity of antennas; an antenna selector operative to select
a subset of antennas from the multiplicity of antennas using a
selection scheme synchronized to the network; a sub-carrier
selector operative to select a set of non-uniformly spaced
sub-carriers from the plurality of sub-carriers using a selection
scheme synchronized to the network; a receiver operative to receive
a plurality of known reference symbols over the selected set of
non-uniformly spaced sub-carriers and through the selected subset
of antennas; a frequency response estimator operative to estimate a
frequency response for each selected sub-carrier over only the
selected subset of antennas; a quantizer operative to quantize the
estimated frequency responses; and a transmitter operative to
transmit the quantized frequency responses to the network via an
uplink control channel.
16. The UE according to claim 15, wherein the multiplicity of
antennas are closely spaced.
17. The UE according to claim 15, wherein the antenna selector is
operative to select a subset of non-uniformly spaced antennas.
18. The UE according to claim 17, wherein the antenna selector is
operative to select the subset of non-uniformly spaced antennas
utilizing a pseudo-random sequence.
19. The UE according to claim 18, wherein the pseudo-random
sequence is a predefined sequence known to both the UE and the
network.
20. The UE according to claim 19, wherein the antenna selector is
operative to pseudo-randomly select a plurality of subsets of
antennas, each at a different time instance.
21. The UE according to claim 20, wherein the antenna selector is
operative to select each subset of antennas based on the known
pseudo-random sequence and a sequence seed corresponding to the
time instance when each subset is selected, wherein the sequence
seed is also known to the network, thereby enabling the network to
determine which antennas are selected by the UE in each subset.
22. A User Equipment (UE) operative in a wireless communication
network, comprising: a multiplicity of antennas; an antenna
selector operative to select a subset of non-uniformly spaced
antennas from the multiplicity of antennas using a selection scheme
synchronized to the network; a receiver operative to receive
through the selected subset of non-uniformly spaced antennas, a
radio signal comprising a plurality of known reference symbols; a
frequency response estimator operative to estimate a frequency
response for each of the selected subset of non-uniformly spaced
antennas; a quantizer operative to quantize the estimated frequency
responses; and a transmitter operative to transmit the quantized
frequency responses to the network via an uplink control channel.
Description
TECHNICAL FIELD
The present invention relates generally to wireless communication
systems, and in particular, the present invention relates to an
efficient system and method of providing channel state information
from user equipment to a wireless communication network.
BACKGROUND
Wireless communication networks transmit communication signals in
the downlink over radio frequency channels from fixed transceivers,
known as base stations, to mobile user equipment (UE) within a
geographic area, or cell. The UE transmit signals in the uplink to
one or more base stations. In both cases, the received signal may
be characterized as the transmitted signal, altered by channel
effects, plus noise and interference. To recover the transmitted
signal from a received signal, a receiver thus requires both an
estimate of the channel, and an estimate of the noise/interference.
The characterization of a channel is known as channel state
information (CSI). One known way to estimate a channel is to
periodically transmit known reference symbols, also known as pilot
symbols. Since the reference symbols are known by the receiver, any
deviation in the received symbols from the reference symbols (once
estimated noise/interference is removed) is caused by channel
effects. An accurate estimate of CSI allows a receiver to more
accurately recover transmitted signals from received signals. In
addition, by transmitting CSI from the receiver to a transmitter,
the transmitter may select the transmission characteristics--such
as coding, modulation, and the like--best suited for the current
channel state. This is known as channel-dependent link
adaptation.
Modern wireless communication networks are interference limited.
The networks typically process transmissions directed to each UE in
a cell independently. Transmissions to other UEs in the same cell
are regarded as interference at a given UE--giving rise to the term
inter-cell interference. One approach to mitigating inter-cell
interference is Coordinated Multipoint (CoMP) transmission. CoMP
systems employ numerous techniques to mitigate inter-cell
interference, including MIMO channels, numerous distributed
antennas, beamforming, and Joint Processing.
Joint Processing (JP) is a CoMP transmission technique currently
being studied for Long Term Evolution (LTE) Advanced. In JP,
transmissions to multiple UEs are considered jointly, and a global
optimization algorithm is applied to minimize inter-cell
interference. That is, JP algorithms attempt to direct transmission
energy toward targeted UEs, while avoiding the generation of
interference at other UEs. To operate effectively, JP systems
require information about the transmission channels. There are two
ways in which the channel information, or CSI, is fed back to
system transmitters: Precoding Matrix Indicator (PMI) and quantized
channel feedback.
PMI feedback, specified in LTE Release 8, is essentially a
recommendation of a transmission format by each UE. A plurality of
pre-defined precoding matrices are designed offline and known at
both the base station and UE. The precoding matrices define various
sets of downlink coding and transmission parameters. Each UE
measures its channel, and searches through the precoding matrices,
selecting one that optimizes some quantifiable metric. The selected
precoding matrix is fed back or reported to the base station. The
base station then considers all recommended precoding matrices, and
selects the precoding and transmission parameters that implement a
globally optimal solution over the cell. In the scenarios
contemplated when Release-8 LTE was designed, PMI feedback works
well, due to a high correlation between recommendations from UEs
and the actual desirable transmission parameters. PMI feedback
compression reduces uplink bandwidth by exploiting the fact that
only part of the channel--the "strong directions," i.e., the signal
space--needs to be fed back to the transmitter.
In JP CoMP applications, it is unlikely that the desired
transmission format (which achieves interference suppression) will
coincide with a transmission format recommended by a UE. No
recommending UE has any knowledge about other UEs that will be
interfered by the transmission to the recommending UE.
Additionally, the recommending UE has no knowledge of transmissions
scheduled to other UEs that will interfere with its signals. Also,
PMI feedback compression reduces bandwidth by reporting only the
part of the channel of interest to transmissions directed to the
recommending UE. While this increases uplink efficiency for
non-cooperative transmission, it is disadvantageous for cooperative
transmission, as it denies the network information about the
channel that may be useful in the JP optimization.
In quantized channel feedback, UEs attempt to describe the actual
channel. In contrast to PMI feedback, this entails feeding back
information about not only the signal space but also the
complementary space (the "weaker space," also somewhat inaccurately
referred to as the "null space") of the channel. Feedback of the
whole channel results in several advantages. With full CSI
available at the network, coherent JP schemes can suppress
interference. Additionally, the network can obtain individualized
channel feedback by transmitting unique reference symbols to each
UE. This enables flexible and future-proof implementations of a
variety of JP transmission methods, since the methods are
essentially transparent to the UE.
Even without JP CoMP transmission, CSI at the network can solve one
of the most fundamental problems plaguing current wireless
system--the inaccuracy in channel-dependent link adaptation due to
the network not being able to predict the interference experienced
by the UEs (a problem closely related to the well-known flash-light
effect). Once the network knows the CSI of bases near each UE, the
network can accurately predict the SINR at each UE resulting in
significantly more accurate link adaptation.
Even though the advantages of direct CSI over PMI feedback are
clear, the major issue with direct CSI feedback is bandwidth. Full
CSI feedback requires a high bitrate to transmit the CSI from each
UE to the network. Time-frequency uplink channel resources must be
used to carry the CSI feedback on the uplink channel, making these
resources unavailable for transmitting user data on the uplink--the
CSI feedback transmissions are thus pure overhead, directly
reducing the efficiency of uplink data transmissions. Conveying
direct CSI feedback to the network without consuming excessive
uplink resources stands as a major challenge of modern
communication system design.
SUMMARY
According to one or more embodiments described and claimed herein,
a UE in a wireless communication network transmits succinct, direct
channel state information to the network, enabling coordinated
multipoint calculations such as joint processing, without
substantially increasing uplink overhead. The UE receives and
processes reference symbols over a set of non-uniformly spaced
sub-carriers, selected according to a scheme synchronized to the
network. The frequency response for each selected sub-carrier is
estimated conventionally, and the results quantized and transmitted
to the network on an uplink control channel. The non-uniform
sub-carrier selection may be synchronized to the network in a
variety of ways.
One embodiment relates to a method of reporting channel state
information by a UE operative in a wireless communication network
in which downlink data is modulated onto a plurality of
sub-carriers, each having a different frequency. A plurality of
known reference symbols are received over a subset of the plurality
of sub-carriers. A set of non-uniformly spaced sub-carriers is
selected using a selection scheme synchronized to the network. A
frequency response is estimated for each selected sub-carrier. The
frequency responses are quantized and transmitted to the network
via an uplink control channel.
Another embodiment relates to a method of reporting channel state
information by a UE operative in a wireless communication network
in which downlink data is modulated onto a plurality of
sub-carriers, each having a different frequency, wherein the UE
includes a multiplicity of antennas. The method includes, at each
iteration, the steps of selecting a subset of antennas from the
multiplicity of antennas using a selection scheme synchronized to
the network; selecting a set of non-uniformly spaced sub-carriers
from the plurality of sub-carriers using a selection scheme
synchronized to the network; and receiving a plurality of known
reference symbols over the selected set of sub-carriers and through
the selected subset of antennas. A frequency response is estimated
for each selected sub-carrier over only the selected subset of
antennas. The estimated frequency responses are quantized and
transmitted to the network via an uplink control channel.
Another embodiment relates to a method of reporting channel state
information by a UE operative in a wireless communication network
in which downlink data is modulated onto a first plurality of
sub-carriers, each having a different frequency, wherein the UE
includes a multiplicity of antennas. The method includes, at each
iteration, the steps of selecting a subset of antennas from the
multiplicity of antennas using a selection scheme synchronized to
the network; receiving a plurality of known reference symbols over
a subset of the first plurality of sub-carriers and through the
selected subset of antennas; and estimating a frequency response
for each of a second plurality of sub-carriers over only the
selected subset of antennas. The method also includes collecting
the estimated frequency responses into a vector; selecting a group
of frequency responses by multiplying the vector of frequency
responses by a mixing matrix that is synchronized to the network,
to yield a vector of selected frequency responses; quantizing the
selected frequency responses; and transmitting the quantized
frequency responses to the network via an uplink control
channel.
Another embodiment relates to a method of reporting channel state
information by a UE operative in a wireless communication network,
wherein the UE includes a multiplicity of antennas. The method
includes, at each iteration, selecting a subset of antennas from
the multiplicity of antennas using a selection scheme synchronized
to the network; receiving through the selected subset of antennas,
a radio signal comprising a plurality of known reference symbols;
estimating a frequency response for each of the selected subset of
antennas; quantizing the estimated frequency response; and
transmitting the quantized frequency responses to the network via
an uplink control channel.
Another embodiment relates to a UE operative in a wireless
communication network in which downlink data is modulated onto a
plurality of sub-carriers, each having a different frequency. The
UE includes one or more antennas; a sub-carrier selector operative
to select a set of sub-carriers using a selection scheme
synchronized to the network; a frequency response estimator
operative to estimate a frequency response for selected
sub-carriers; a quantizer operative to quantize the estimated
frequency responses; and a transmitter operative to transmit
selected quantized frequency responses to the network via an uplink
control channel.
In a further embodiment of the UE, the one or more antennas
comprise a multiplicity of antennas, and the UE also includes an
antenna selector operative to select a subset of antennas from the
multiplicity of antennas using a selection scheme synchronized to
the network, and a receiver operative to receive a plurality of
known reference symbols over the selected set of sub-carriers and
through the selected subset of antennas. The frequency response
estimator in this embodiment is operative to estimate the frequency
response for each selected sub-carrier over only the selected
subset of antennas.
Another embodiment relates to a UE operative in a wireless
communication network and including a multiplicity of antennas; an
antenna selector operative to select a subset of the multiplicity
of antennas using a selection scheme synchronized to the network; a
receiver operative to receive through the selected subset of
antennas, a radio signal comprising a plurality of known reference
symbols; a frequency response estimator operative to estimate a
frequency response for each of the selected subset of antennas; a
quantizer operative to quantize the estimated frequency responses;
and a transmitter operative to transmit selected quantized
frequency responses to the network via an uplink control
channel.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of a wireless communication
network.
FIG. 2A is a time-frequency plot showing reference symbol
transmission from a single antenna port.
FIG. 2B is a time-frequency plot showing reference symbol
transmission from two antenna ports.
FIG. 2C is a time-frequency plot showing reference symbol
transmission from three antenna ports.
FIG. 3 is a flow diagram of a first exemplary embodiment of a
method of reporting CSI feedback by a UE.
FIG. 4A is a graph of the in-phase component of a representative
channel response, depicting the quantized channel estimates
reported to the network.
FIG. 4B is a graph of the quadrature component of a representative
channel response, depicting the quantized channel estimates
reported to the network.
FIG. 5 is a flow diagram of a second exemplary embodiment of a
method of reporting CSI feedback by the UE.
FIG. 6 is a simplified block diagram of an exemplary embodiment of
a UE modified in accordance with the present disclosure.
FIG. 7 is a flow diagram of a third exemplary embodiment of a
method of reporting CSI feedback by the UE.
DETAILED DESCRIPTION
For the purpose of clear disclosure and full enablement, the
present invention is described herein as embodied in a wireless
communication network based on Orthogonal Frequency Division
Multiplex (OFDM) modulation. More specifically, embodiments herein
are based on the Evolved Universal Terrestrial Radio Access
(E-UTRA) system, which is also commonly referred to as the
Long-Term Evolution (LTE) of the widely deployed WCDMA systems.
Those of skill in the art will readily appreciate that these
systems are representative only and not limiting, and will be able
to apply the principles and techniques of the present invention to
a wide variety of wireless communication systems, based different
access and modulation methods, given the teachings of the present
disclosure.
FIG. 1 depicts a wireless communication network 10. The network 10
includes a Core Network (CN) 12, communicatively connected to one
or more other networks 14, such as the Public Switched Telephone
Network (PSTN), the Internet, or the like. Communicatively
connected to the CN 12 are one or more Radio Network Controllers
(RNC) 16, which in turn control one or more Node B stations 18. The
Node B 18, also known as a base station, includes radio frequency
(RF) equipment and antennas necessary to effect wireless radio
communications with one or more user equipment (UE) 20 within a
geographic region, or cell 22. As depicted, the Node B 18 transmits
data and control signals to the UE 20 on one or more downlink
channels, and the UE similarly transmits data and control signals
to the Node B 18 on the uplink.
Interspersed within the data on the downlink transmission, the
network 10 transmits reference symbols, also known in the art as
pilot symbols, to assist the UEs 20 performing channel estimation
on the downlink channel responses. FIG. 2A depicts example of the
reference symbol resources for the LTE network 10 of FIG. 1, when
the Node B 18 transmits on a single antenna port. The depicted grid
plots sub-carriers on the ordinate axis (frequency increasing
downwardly) and time (increasing to the right) on the abscissa
axis. Note that the time periods are organized into frames, with
even-numbered and odd-numbered slots depicted. Each grid element is
an OFDM time-frequency resource element, which may carry a data
symbol, reference symbol, or neither. FIGS. 2B and 2C depict
reference symbol transmissions when the Node B 18 transmits on two
and four antenna ports, respectively.
The reference symbols enable the UE to employ a wide range of
standard techniques to estimate the frequency responses of all
sub-carriers. Since the values of the reference symbols are known
to the UE 20, the estimation quality is generally highest on the
sub-carriers occupied by the reference symbols.
FIG. 3 depicts a method of reporting CSI by a UE 20 to the network
10, according to one embodiment. The UE 20 receives known reference
symbols over some of the sub-carriers transmitted to it, as
depicted in FIG. 2 (block 102). The UE 20 selects a set of
non-uniformly spaced sub-carriers, on which to perform channel
estimation for CSI feedback (block 104). In one embodiment,
selection of sub-carriers is limited to those on which reference
symbols are transmitted, because the channel estimation quality is
generally highest at these sub-carriers. However, in other
embodiments, the UE 20 additionally selects one or more
sub-carriers that do not include reference symbols. As discussed
further herein, the selection of non-uniformly spaced sub-carriers
is performed according to a scheme that is synchronized in some
manner with the network. The UE 20 estimates the frequency response
of the channel (block 106). The frequency response samples
associated with the selected sub-carriers are then quantized or
encoded by a suitable source encoder into digital bits (block 108).
The digital bits are then transmitted via a suitable control
channel from the UE 20 to the network 10 (block 110). The control
channel provides suitable error detection and correction coding as
well as radio resources (transmission power and frequency resource
allocation) to ensure proper reception quality at the network 10.
The method then repeats.
FIGS. 4A and 4B depict the representative channel frequency
response for in-phase (FIG. 4A) and quadrature (FIG. 4B) components
of a received signal. Out of 50 reference symbol sub-carriers tones
over a 5 MHz band, a subset of 15 non-uniformly spaced reference
symbol sub-carriers is selected. These samples are depicted as
stars in FIGS. 4A and 4B. The samples do not always lie on the
frequency response curves due primarily to two sources of noise.
First, it is assumed the mean square error (MSE) of the UE channel
estimator at the reference symbol sub-carriers is .about.20 dB.
Secondly, the I and Q parts of the selected channel estimates are
digitized independently by a simple 4-bit uniform quantizer. The
resulting average quantization noise is about .about.22 dB. With
this setup, a total of 15*4*2=120 bits are fed back by the UE
20.
With uniform sampling, the Nyquist theorem dictates that samples
(sub-carriers) must be selected at twice the highest frequency of
the channel frequency response curve, to fully characterize the
curve. Using non-uniform samples, however, far fewer than the
Nyquist criterion of sub-carriers may be selected, with a high
probability of accurate reconstruction of the channel frequency
response curve by the network 10. Accordingly, by selecting
non-uniformly spaced sub-carriers, the UE 20 may fully characterize
the channel and provide direct CSI feedback, without imposing
excessive overhead on the uplink channel.
Upon receipt by the network 10, the received CSI feedback bits are
demodulated and inverse quantized. The complete frequency domain
channel coefficients may be estimated by setting a time-domain
tap-delay channel model based on the received sub-carrier samples.
Applying, e.g., a Fast Fourier Transform (FFT) to the estimated
delay coefficients yields a frequency-domain response very close to
that depicted in FIGS. 4A and 4B. Detail of the network-side
processing of CSI feedback based on channel estimates of
non-uniformly spaced sub-carriers is disclosed in copending U.S.
patent application Ser. No. 12/555,973, assigned to the assignee of
the present application, filed concurrently herewith, and
incorporated herein by reference in its entirety. The network-side
processing assumes the network 10 is aware of which non-uniformly
selected sub-carriers were analyzed by the UE 20. Thus, the UE 20
must select the non-uniformly spaced sub-carriers according to a
scheme, protocol, or formula that is synchronized with the network
10. There are numerous ways to accomplish this.
In one embodiment, the set of non-uniformly spaced sub-carriers is
changed for each batch, or iteration, of CSI feedback reporting, in
a manner coordinated with the network 10.
In one embodiment, the set of non-uniformly spaced sub-carriers is
selected based on pseudo-randomized indices with synchronized
reading offset. For example, the pseudo-randomized indices may be
obtained by taking sequentially the indices produced by a
pseudo-random number generator. The pseudo-random number generator
may be computed based on an algebraic modification of the input
reading indices. For example, the algebraic modification may be
based quadratic permutation polynomials (QPP), as described in the
3GPP Technical Specifications 36.212, "Multiplexing and channel
coding," incorporated herein by reference. As another example, the
algebraic modification may be based on a finite-field
computation.
As another example of pseudo-randomized indices with synchronized
reading offset, the pseudo-randomized indices may be obtained by
taking sequentially the indices produced by an interleaver. The
interleaver may be computed based on a column-interleaved
rectangular array, as described in section 5.1.4.2.1 of the 3GPP
Technical Specifications 36.212.
As yet another example, the sequential reading of indices may be
synchronized between UE 20 and the network 10 via an agreed index
reading offset. The agreed index reading offset may be obtained in
many ways. It may be transmitted explicitly in the same transport
channel with the digital bits from the UE 20 to the network 10.
Alternatively, the agreed index reading offset may be implicitly
computed based on a UE 20 identification number, a sub-frame
number, a CSI feedback batch or iteration count, an antenna
identification number, a network-side identification number, or the
uplink control channel recourse index (e.g., where it is the index
of the first resource block for the uplink control channel). The
agreed index reading offset may be implicitly computed based on the
downlink control channel resource index (e.g., where it is the
index of the first resource block for the downlink control
channel). Alternatively, the agreed index reading offset may be
transmitted from the network 10 to the UE 20 prior to the UE 20
performing channel estimation, or may be pre-agreed between the
network 10 and the UE 20. In either case, the index reading offsets
may be stored in the UE 20 as a look-up table.
In one example, the set of non-uniformly spaced sub-carriers is
selected by initially selecting uniformly spaced sub-carriers, and
then applying pseudo-randomized dithering, with a key synchronized
to the network 10, to the uniformly spaced sub-carriers to generate
the set of non-uniformly spaced sub-carriers. In one embodiment,
the maximum span of the pseudo-randomized dithering is selected to
be smaller than the uniform spacing in the uniformly spaced
indices. The generation of pseudo-randomized dithering may be
computed based on algebraic modification of an input key. As
described above with respect to the non-uniform sub-carrier
selection, the pseudo-randomized dithering may be obtained by
taking sequentially the indices produced by an interleaver or a
pseudo-random number generator, with the generation of indices
computed by the UE 20 based on any of the factors above, or
communicated between the network 10 and UE 20, as also described
above.
A more general formulation of the selection of sub-carriers,
channel estimation, and quantization and reporting of CSI feedback
is now presented. The frequency response of a channel at frequency
f and time t can be expressed in terms of the time domain channel
taps h(l;t) having delays .tau..sub.i as follows:
.function..times..function..times.e.times..times..times..pi..times..times-
..times..times..tau. ##EQU00001## At each reporting iteration or
time t, the following steps are performed by the UE 20:
First, the UE 20 forms an estimate of the downlink channel at a
number of sub-carriers. As described above, known reference signals
are transmitted from each network antenna (see FIGS. 2A-2C), and
the UE 20 can use these reference signals to form an estimate of
the channel at a number of sub-carriers using standard techniques.
These estimates are denoted by the following N.times.1 vector:
g(t)=[H(f.sub.1;t)H(f.sub.2;t) . . . H(f.sub.N;t)].sup.T Where
H(f;t) is the UE-estimated frequency response of the channel at
frequency f and time t.
Second, for each reporting instant, the UE 20 forms a number of
linear combinations of elements of g(t), i.e., the UE 20 multiplies
the vector g(t) by a mixing matrix P(t), of size M.times.N, to get
a new vector r(t) of size M.times.1, according to:
r(t)=P(t).times.g(t). In embodiments where the elements of P(t)
comprise only the values zero or one, P(t) "selects" elements from
the vector of channel estimates of non-uniform sub-carriers g(t)
according to each row of P(t). In some embodiments, the results of
computations or communications described above to select the
reading offsets of pseudo-randomized indices may be stored in the
mixing matrix P(t). In more general embodiments, however, the
elements of P(t) are not restricted to the values zero or one. For
example, the elements may comprise fractional values between zero
and one, in which case they act as weighting factors as well as
selectors. Additionally, the elements may comprise complex
values.
The mixing matrix P(t) may be changed for different sets of
iterations of CSI feedback. In one embodiment, the selection of
P(t) may be by a round-robin selection from among a collection of
mixing matrices. In one embodiment, the changing of P(t) may
comprise selecting different row compositions. For example, the
selection of different row compositions may be based on the
round-robin use of a plurality of rows. As another example, it may
be based on a pseudo-randomized selection from a plurality of rows.
The pseudo-randomized selection of rows may be obtained by taking
sequentially the indices produced by an interleaver or
pseudo-random number generator, where the indices may be
communicated or computed in any manner described above.
In one embodiment, the mixing matrix P(t) comprises rows having at
most one non-zero element. In another embodiment, the mixing matrix
P(t) comprises rows given by an orthonormal matrix, such as a
Hadamard matrix. In yet another embodiment, the mixing matrix P(t)
comprises rows given by a unitary matrix. In still another
embodiment, the mixing matrix P(t) may be generated by first
generating pseudo-random matrices {A.sub.i} with independent
Gaussian-distributed entries, performing a QR decomposition on each
A.sub.i, and using each resulting unitary Q matrix as a candidate
for P(t).
However the mixing matrix P(t) is derived, after the multiplication
with g(t), the elements of the product matrix r(t) are quantized
using a quantizer Q.sub.r() to obtain a number of bits, denoted as
the vector b(t), representing the vector r(t). The bits in b(t) are
then transmitted to the network 10 using an uplink control channel.
As known in the art, the transmission process may include adding
redundancy such as CRC, FEC, and the like, to ensure reliable
transmission to the network 10.
In the embodiments described above, the UE 20 determines the
parameters for selecting non-sequential sub-carriers and/or the
dithering parameters to generate a non-sequential selection of
sub-carriers, such as the selection of indices for a pseudo-random
number generator, autonomously or quasi-autonomously from the
network 10 (although, of course, whatever selection mechanism is
employed must be synchronized with the network 10). In some
embodiments, however, the network 10 directly controls these and
other parameters via transmissions to the UE 20 in the
downlink.
In one embodiment, the network 10 determines the set of
sub-carriers (f.sub.1, . . . , f.sub.N), for which the UE 20 should
estimate channel response and place in vector g(t). In one
embodiment, the network 10 determines the mixing matrix P(t) that
the UE 20 should use at each reporting instance. In one embodiment,
the network 10 determines the quantizer Q.sub.r() that the UE 20
uses at each reporting instance, which determines, for example, how
many bits are used to quantize each element of r(t). In one
embodiment, the network 10 determines how often the CSI feedback
reports should be transmitted by the UE 20 on the uplink. In all
these embodiments, the network 10 communicates the relevant
determinations to the UE 20 in downlink communications.
Additionally, of course, the network 10 schedules the
time-frequency uplink resources on which the CSI feedback reports
shall be transmitted by the UE 20, just as for any uplink
communications.
In a typical network 10, each UE 20 might have to report CSI
feedback on multiple downlink channels, from multiple different
Node Bs 18. Since the path loss between each UE 20 and Node B 18 is
different, the downlink channels to be estimated and reported by
each UE 20 will have different average power. With a fixed bitrate
budget for CSI feedback allocated to each UE 20, a problem arises
as how this total fixed bitrate should be divided among the
different downlink channels seen by the UE 20.
If a channel between a given UE 20 and a given Node B 18 is
extremely weak, the signals transmitted from the Node B 18 will
have very little impact at the receiver of the UE 20. Hence, there
is little need for the UE 20 to report CSI feedback corresponding
to the channels that are very weakly received at the UE 20.
Accordingly, in one embodiment, the UE 20 allocates a larger
portion of its allocated CSI feedback bitrate to the downlink
channels that are relatively strong, than to the channels that are
relatively week. Given a set of average channel signal strengths
g(1), g(2), . . . , g(B), and a total CSI feedback allocation of K
bits, the network 10 can allocate its total bitrate budget among
the various channels. In one embodiment, the network 10 performs
the allocation according to a Generalized Breiman, Friedman,
Olshen, and Stone (BFOS) Algorithm, as described by E. A. Riskin in
the paper, "Optimal Bit Allocation via Generalized BFOS Algorithm,"
published in the IEEE Trans. Info. Theory, 1991, the disclosure of
which is incorporated herein by reference in its entirety.
In one embodiment, the reporting of CSI feedback may be spread out
over a plurality of iterations of CSI feedback. That is, a set of
non-uniformly spaced subcarriers are selected, and a frequency
response is calculated for each subcarrier. The frequency responses
are quantized. However, rather than transmit all of the quantized
frequency response data to the network at once, the reporting is
spread over two or more iterations of CSI feedback. For example, at
time N, some number, e.g. ten, subcarriers are selected, and their
frequency responses calculated and quantized (possibly jointly).
The quantized bits may then be transmitted to the network over the
next ten time intervals, e.g., at times N+1, N+2, . . . , N+10. Of
course, reports for two subcarriers could be transmitted at a time,
using five CSI reporting intervals, or any other permutation. This
reporting method minimizes the uplink bandwidth required for
reporting CSI captured at one time.
In another embodiment, a persistent form of CSI reporting comprises
selecting one or more subcarrier and calculating its frequency
response. The quantized frequency response is then transmitted to
the network. Over time, the selection of subcarriers is
non-uniform. For example, a first subcarrier is selected at time N,
and its quantized frequency response is transmitted to the network
at reporting interval N+1. At that time, a new subcarrier (at a
different frequency) is selected, and its quantized frequency
response is transmitted to the network at the reporting interval
N+2. Similarly, two or more subcarriers may be selected during any
given CSI generation and reporting interval. This reporting method
minimizes the uplink bandwidth by spreading both the subcarrier
selection, and the reporting of quantized CSI data, over time.
In 5G systems, which will likely be deployed in high frequency
bands, the signal wavelength is much smaller than existing systems,
which means the antennas can be packed closely together, and each
device (e.g., UE) can have a massive number of antennas (for
example, several tens or even hundreds) depending on the capability
and size of the UE. The number of antennas may typically range, for
example, from a 4.times.4 array (i.e., 16 antennas) to a
16.times.16 array (i.e., 256 antennas). In this case, feeding back
a quantized received signal (or frequency responses of such signal)
for every antenna or for most of the antennas will create a large
amount of overhead. So it would be advantageous to extend the
method of the present disclosure to the spatial domain, where a
relatively small subset of antenna elements (for example, 5 to 30
percent of the total) is selected to receive the signal and
ultimately produce a quantized frequency response for feedback to
the transmitter. The selections of frequency subcarriers in the
frequency domain and the selections of antenna subsets in the
spatial domain may be performed together or independently of each
other.
In this embodiment, the UE 20 transmits succinct, direct channel
state information to the network, enabling coordinated multipoint
calculations such as joint processing, without substantially
increasing uplink overhead. The UE, which has a multiplicity of
antennas, receives and processes reference symbols over a small
selected subset of antennas. The frequency response for each
selected subset of antennas may be estimated conventionally, and
the results are quantized and may be transmitted to the network on
an uplink control channel.
The number of antennas in the small selected subset depends on the
scattering environment in which the UE is located, and the
closeness of the antenna spacing. As noted above, a range between 5
to 30 percent of the total number of antennas should account for
most scattering environments and antenna designs.
The antennas may be selected for the subset pseudo-randomly
(according to a predetermined pseudo-random sequence) or regularly
(i.e., uniformly). Pseudo-random selection allows fewer antennas to
be selected for the same performance, but requires a more
computationally intensive algorithm for reconstruction of the
channel estimate at the network. Nevertheless, pseudo-random
selection may be preferred since it implies a non-uniform spacing
in the selected antenna subset, which provides better results than
uniform spacing. Note that similar to the pseudo-random selection
process in the frequency domain described above, the predetermined
pseudo-random sequence used for antenna selection must be known to
the network, a priori, and can be provisioned there in any of a
number of known ways.
It should be noted that it is not necessary to sample all of the
antennas. The number of subsets depends on the pseudo-random
sequence used for sampling. When both the network and the UE know
the pseudo-random sequence and the corresponding "seed" of the
sequence is used, both the network and the UE can increment the
"seed" once an antenna (or frequency subcarrier) is randomly
selected.
FIG. 5 is a flow diagram of a second exemplary embodiment of a
method 200 of reporting CSI feedback by the UE 20. The UE 20
selects a subset of antennas from the multiplicity of UE antennas
using a selection scheme synchronized to the network (block 202).
The UE selects a set of non-uniformly spaced sub-carriers, on which
to perform channel estimation for CSI feedback using a selection
scheme synchronized to the network (block 204). In one embodiment,
selection of sub-carriers is limited to those on which reference
symbols are transmitted, because the channel estimation quality is
generally highest at these sub-carriers. However, in other
embodiments, the UE additionally selects one or more sub-carriers
that do not include reference symbols. As discussed further herein,
the selection of non-uniformly spaced sub-carriers is performed
according to a scheme that is synchronized in some manner with the
network.
The UE receives known reference symbols over the selected set of
sub-carriers transmitted to it, as described above, and through the
selected subset of antennas (block 206). The UE estimates the
frequency response for each selected sub-carrier for the selected
subset of antennas (block 208). The frequency response samples
associated with the selected sub-carriers and antennas are then
quantized or encoded by a suitable source encoder into digital bits
(block 210). The digital bits are then transmitted via a suitable
control channel from the UE to the network 10 (block 212). The
control channel provides suitable error detection and correction
coding as well as radio resources (transmission power and frequency
resource allocation) to ensure proper reception quality at the
network 10. The method then repeats for additional sets of
sub-carriers and additional subsets of antennas.
The antenna selection at block 202 may dictate which signals (each
from one antenna) are sampled and processed by the receiver in
block 202. In one embodiment, selected antennas may be connected to
a limited number of receiving chains. So depending on the subset of
antennas being selected, block 202 processes the signals only from
those antennas. In an alternative embodiment, signals from all of
the antennas are processed into digital signals first, and the
antenna selection then dictates which signals are discarded and
which ones are retained and processed for feedback to the
network.
By the same reasoning, the selection of the set of non-uniformly
spaced sub-carriers at block 204 may be done prior to the receiver
processing at block 206 (after, or in parallel with block 202),
since it is useful to know which set of sub-carriers are selected
before receiving from those sub-carriers. In other words, both
spatial and frequency domain selection may be done first, and then
the UE processes only those signals (or reference symbols) from the
selected antennas and sub-carriers. In another alternative
embodiment, the UE receives over a larger set of sub-carriers first
and then discards all but those selected to be processed by block
208 and forward.
FIG. 6 is a simplified block diagram of an exemplary embodiment of
a UE 20 modified in accordance with the present disclosure.
Operation of the UE may be controlled, for example, by one or more
processor(s) 220 coupled to a memory 222 that stores computer
program instructions. The processor controls the components of the
UE when the processor executes the computer program instructions.
In different embodiments, the UE may perform (1) frequency domain
selection of a set of sub-carriers for further processing; (2)
spatial domain selection of a subset of antennas for further
processing, or (3) both frequency domain and spatial domain
selections.
A multiple antenna array 224 receives a downlink signal with
reference symbols. For illustrative purposes, an example array may
include 100 antennas arranged in a 10.times.10 planar array. The
received symbols are passed to a receiver (RX) 226. In an
embodiment of the UE operating in only the frequency domain, a
sub-carrier selector 228 selects a set of non-uniformly spaced
sub-carriers using a selection scheme synchronized to the network,
and provides this information to the receiver. A frequency response
estimator 230 estimates a frequency response for each selected
sub-carrier, and a quantizer 232 quantizes the estimated frequency
response. A transmitter (TX) 234 then transmits the quantized
frequency responses to the network via an uplink control
channel.
In an embodiment of the UE operating in both the frequency domain
and the spatial domain, the sub-carrier selector 228 informs the
receiver of the set of non-uniformly spaced sub-carriers, and an
antenna selector 236 selects a subset of antennas from the array
224 using a selection scheme synchronized to the network, and
informs the receiver 226. In the example embodiment having a
10.times.10 antenna array, the antenna selector may be a
pseudo-random selector that, at each time instance, utilizes a
predefined pseudo-random sequence to sample each of the two
dimensions of the array with, for example, 30% probability of
selection. So three out of ten antennas in one of the dimensions
are selected, and then for each of these three selected antennas,
three out of ten antennas in the other dimension are selected.
Thus, a total of nine antennas out of the 100 total antennas are
selected as the subset. Both the UE and the network know which
antennas are selected since they both know the pseudo-random
sequence used for the random selection and the corresponding "seed"
being used at the time instance.
The frequency response estimator 230 estimates a frequency response
for each selected sub-carrier for the selected subset of antennas.
The quantizer 232 quantizes the estimated frequency responses, and
the transmitter 234 then transmits the quantized frequency
responses from each of these nine selected antennas to the network
via the uplink control channel.
In an embodiment of the UE operating in only the spatial domain,
the antenna selector 236 selects a subset of antennas from the
array 224 using a selection scheme synchronized to the network and
informs the receiver 226, which samples and processes only signals
from the selected antennas. The frequency response estimator 230
estimates a frequency response for each of the selected antennas.
The quantizer 232 quantizes the estimated frequency responses, and
the transmitter 234 then transmits the quantized frequency
responses for each of the selected antennas to the network via the
uplink control channel.
FIG. 7 is a flow diagram of a third exemplary embodiment of a
method 300 of reporting CSI feedback by the UE 20. The UE 20
selects a subset of antennas from the multiplicity of UE antennas
using a selection scheme synchronized to the network (block 302).
The UE receives known reference symbols over the selected subset of
antennas (block 304). The UE estimates the frequency response for
each antenna in the selected subset of antennas (block 306). The
frequency response estimates are then quantized or encoded by a
suitable source encoder into digital bits (block 308). The digital
bits are then transmitted via a suitable control channel from the
UE to the network 10 (block 310). The control channel provides
suitable error detection and correction coding as well as radio
resources (transmission power and frequency resource allocation) to
ensure proper reception quality at the network 10. The method then
repeats for additional subsets of antennas.
Embodiments described herein significantly reduce CSI feedback
bandwidth, while enabling highly accurate CSI availability to the
network. This efficiently allows for the implementation of advanced
network protocols such as joint processing in coordinated
multipoint transmission, without consuming excess uplink
transmission resources.
The present invention may, of course, be carried out in other ways
than those specifically set forth herein without departing from
essential characteristics of the invention. The present embodiments
are to be considered in all respects as illustrative and not
restrictive, and all changes coming within the meaning and
equivalency range of the appended claims are intended to be
embraced therein.
* * * * *